This revised application proposes to investigate the regulation of the cell cycle in trophoblast cells prior to differentiation, which determines the available placenta. Specifically, she proposes to 1: establish the relationship between cyclin E expression, levels of p21, p27, and p57 kinase activity and normal placental development using immunoprecipitations and western blot analyses from isolated cell preparations; 2: evaluate effects of TGFb1, which inhibits trophoblast proliferation and promotes differentiation, on the cyclin/ inhibitor ratios and kinase activity and associated cytotrophoblast differentiation; 3: determine if the cytotrophoblast differentiation program can be altered by manipulating the cyclin:inhibitor ratio in culture.